Document plus addressing and DEFAULT_RELAY_HOST format
Document plus addressing and DEFAULT_RELAY_HOST format

These are a couple of things that work but got me losing lots of hours. Let me add them to README:

1. Fix #1330 explaining that plus addressing works out of the box.
2. Explain that DEFAULT_RELAY_HOST will fail to authenticate with RELAY_USER and RELAY_PASSWORD if you forgot the brackets.

@Tecnativa TT20505

tomav/docker-mailserver,tomav/docker-mailserver,tomav/docker-mailserver